1. Field of the Invention
The present invention relates to motors having a stator section equipped with first and second stator assemblies and a rotor section equipped with first and second permanent magnets respectively disposed opposite to the first and second stator assemblies.
2. Description of the Related Arts
Motors having a stator section equipped with first and second stator assemblies and a rotor section equipped with first and second permanent magnets respectively disposed opposite to the first and second stator assemblies are generally called PM (permanent magnet) type stepping motors.
The range of application of such PM type stepping motors has expanded as actuators for a variety of equipment because of their excellent controllability. In recent years, PM type stepping motors have been used in electronic equipment such as video cameras and digital cameras. In this connection, thinner and more miniaturized motors are in demand along with advances that are being made to improve the performance of various equipment.
One example of the type of motors described above is shown in FIG. 5. For example, a motor 50 shown in FIG. 5 has a stator section 51 equipped with a first stator assembly 52 and a second stator assembly 53, and a rotor section 54 equipped with first and second permanent magnets 56 and 57 respectively disposed opposite to the first and second stator assemblies 52 and 53.
The first and second permanent magnets 56 and 57 are generally formed by compression molding, which achieves a higher density than magnets formed by injection molding. Accordingly, the first and second permanent magnets 56 and 57 formed by compression molding can provide an excellent magnetic characteristic, better than magnets that are formed by injection molding.
Also, as shown in FIG. 5, the first and second permanent magnets 56 and 57 are affixed to a rotation shaft 55, with a gap 60 provided between them. More specifically, the first permanent magnet 56 is disposed opposite to pole teeth 521a and 523a of first outer and inner stator cores 521 and 523 that form the first stator assembly 52, and the second permanent magnet 57 is similarly disposed opposite to pole teeth 531a and 533a of second outer and inner stator cores 531 and 533 that form the second stator assembly 53.
By disposing the first and second permanent magnets 56 and 57 separated from each other, magnetic paths are not formed or difficult to be formed between the first permanent magnet 56 and the second outer and inner stator cores 531 and 533, or between the second permanent magnet 57 and the first outer and inner stator cores 521 and 523. As a result, leaks of magnetic fluxes are prevented, and the rotation performance of the motor 50 can be improved.
An adhesive retaining concave section 57a is formed at one end of the second permanent magnet 57. When the rotation shaft 55 and the second permanent magnet 57 are bonded with adhesive, an excess portion of the adhesive applied to the rotation shaft 55 is retained in the adhesive retaining concave section 57a, and the retained excess portion of the adhesive enhances the bonding force with the rotation shaft 55.
Also, a circular concave section 57b is formed at the other end of the second permanent magnet 57 around the rotation shaft 55 to receive a bearing section 58a of a radial bearing 58, in order to secure an effective bearing length, and reduce the thickness and the size of the motor 50.
However, if the motor size and the thickness of the motor 50 are reduced, the outer diameter of the rotation shaft 55 to which the first and second permanent magnets 56 and 57 are bonded becomes smaller, and the measurements in the axial direction of the first and second permanent magnets 56 and 57 become shorter. As a result, bonding areas of the permanent magnets 56 and 57, which are formed between inner circumferential surfaces of the first and second permanent magnets 56 and 57 and the outer circumferential surface of the rotation shaft 55, are reduced. The reduction in bonding areas leads to problems in that a sufficient bonding force cannot obtained between the first and second permanent magnets 56 and 57 and the rotation shaft 55, and the required perpendicularity of the first and second permanent magnets 56 and 57 with respect to the rotation shaft 55 cannot be obtained. As a result, variations occur in the gap between the outer circumferential surfaces of the first and second permanent magnets 56 and 57 and their opposing pole teeth 521a and 523a, and 531a and 533a. Consequently, variations are caused in the magnetic attraction force and magnetic repelling force between the first and second permanent magnets 56 and 57 and their opposing pole teeth 521a and 523a, and 531a and 533a, which results in problems of irregular rotation speed and deteriorated rotation performance of the motor 50.
Moreover, in the motor 50, the second permanent magnet 57 is formed with the circular concave section 57b to receive the bearing section 58a of the radial bearing 58 and the adhesive retaining concave section 57a to retain an excess portion of adhesive. As a result, a bonding overlap width W11 of the second permanent magnet 57 with respect to the rotation shaft 55 is substantially reduced. Accordingly, although the adhesive retaining concave section 57a that enhances the bonding force is formed, it is difficult to obtain a sufficient bonding force or a required perpendicularity.
In order to secure a long bonding overlap width W11, the first and second permanent magnets 56 and 57 may be formed in one piece without being separated into a single permanent magnet, and the single permanent magnet may be bonded to the rotation shaft 55 to form a rotor section. If this structure is adapted, leaks of magnetic fluxes, which may occur between the first permanent magnet 56 and the second outer and inner stator cores 531 and 533 and between the second permanent magnet 57 between the first outer and inner stator cores 521 and 523 as described above, need to be prevented. To achieve this, for example, a concave section may be formed in the outer circumferential surface of the permanent magnet in a portion that corresponds to the gap 60 in FIG. 5.
However, the permanent magnet is formed by a compression molding method in which a mixture of magnetic powder of high density and binder is filled in a metal mold of a compression molding machine and compressed by a press machine, thereby hardening the molded body.
Due to the physical property of the mixture, its flow ability is poor, and therefore a problem arises in that a metal mold having a complex shape cannot be completely filled with the mixture, and it is difficult for the press machine to apply a uniform pressure to areas formed in the concave section.
To address the problems described above, when a concave section is to be formed in the outer circumferential surface of the permanent magnet, a cylindrical body having a uniform outer diameter may first be formed by compression molding, and then a required portion for the concave section is formed by cutting. However, this method leads to problems of an increased number of manufacturing steps and a higher cost.